Process for making inactive menthol



Patented Sept. 16, 1930 UNITED STATES PATENT orrica KARL SCHGLLKOPF, OF D'USSELDORF-OBERKASSEL, AND ARTHUR SERIIII, OF DUSSEL- DORF-HEERDT, GERMANY, ASSIGNORS TO EI-IEINISCHE KAMPFER-FABRIK GESELL- SCI-IAFT MIT BESGI-LRANKTER EAF'IUNG, OF .DUSSELDORF-OBERKASSEL, GERMANY PROCESS non MAKING nncrrvn MENTHOL No Drawing. Application filed June 29, 192?, Serial No. 202,458, and in Germany December 15, 1926.

The present invention refers to the manufacture of inactive menthol (the racemate of natural menthol) from a new isomer of inactive menthol by a catalytic transformation (isomerization) c The new isomer of inactive menthol shall be called solid inactive isomenthol, for it is an alcohol of the inactive isomenthone (the semicarbazone of the latter having the meltingpoint of 217 degrees centigrade). The solid .inactiv'e isomenthol is characterized by its 1nelting-point of 5253 degrees centigrade, its boiling-point of 218 degrees centigrade under 760 mm. pressure, and its acid phthalic ester havingthe melting-point of 113415 degrees Centigrade, and its paranitro-benzoic ester having the melting-point of 65 degrees centigrade. The solid inactive isomenthol is devoid of the smell and taste of the inactive menthol (the racemate of natural menthol).

The solid inactive isomenthol, described partly in our copending application 146,564: filed November 5th, 1926, is obtained by subjecting the synthetically obtained mixtures of the various inactive isomeric methols to a fractional distillation, viz, separating the fraction boiling between 212214l degrees centigrade, containing inactive neomenthol, separating the second fraction bolhng between 215-217 degrees centigrade at T mm. pressure containing inactive menthol and separating the third fraction boiling at 218 degrees Centigrade containing solid inactive isomenthol.

Solid inactive isomenthol can also be ob tained by subjecting crude inactive mentholobtained for instance by a freezing out proc ess from the synthetically obtained menthols mixturesto a fractional distillation, Whereby the solid inactive isomenthol is'separated as highest boiling fraction.

The solid inactive isomenthol can also be obtained by saponifying the mother liquors of crystallization left in purifying the still slightly impure inactive menthol by conversion into the crystalline esters according to the process described in U. S. Patent No. 1,672,346, and subjecting the menthols mixture obtained by saponifying-in Which the solid inactive isomenthol preponderates-to fractional distillation, whereby the solid inactive isomenthol is separated as the fraction of the highest boiling point (218 degrees centigrade at 760 mm. pressure). c

The slightly impure solid inactive isomenthol can be obtained in pure state, meltingpoint 5253 degrees centigrade, by another fractional distillation. The pure solid inactive isomenthol can also be obtained from the still slightly impure solid inactive isomenthol byconversion of the latter into the crystalline esters, purifying the esters by crystallization and saponifying the'pure esters.

The solid inactive isomenthol prepared by the described processes or in any other Way. may be transformed into inactive menthol by a catalytic isomerization of the solid inactive isomenthol. 7

Suitable catalystsfor the isomerization according to the present process are the same as for hydrogenation, for example, nickel, cobalt, copper, platinum, palladium or mixtures of these metals in a finely divided condition or distributed oncarriers such as kieselguhr, asbestos, pumice stone, barium sulphate and the like. In place of the metals their oxygen compounds or their readily decomposable carhon-containing salts, such as carbonates and formates may also be employed.

The transformation can be efl'ected by means of hydrogenating catalysts in presence of hydrogen under ordinary or raised pressure With the use of heat and agitation, and separating the crude inactive menthol from the resulting mixture by freezing-out or fractional distillation by physical methods described in U. S. Patent No. 1,625,771 for example, purifying the crude inactive menthol by Way of its esters, and transforming the residual isomers anew. The solid inactive isomenthol may also be added to fresh quantities of thymol, inactive menthone or isomenthone or inactive piperitone (A-p-menthen-3- one), for hydrogenation and simultaneous trasformation. Again, the transformation of the solid inactive isomenthol may be efiected simultaneously with admixture of other isomers of inactive menthol.

The solid inactive isomenthol may also be converted into the corresponding ketone it had inactive isomenthoneby oxidation with chromic acid according to Beckmanns proc ess (Annalen der Chemie, vol. 250, p. 325) or may also be converted into a mixture of inactivc menthone and isomenthone by dehydrogenation with the assistance of the same catalysts as for hydrogenation, but at higher temperatures than for the hydrogenation, and the said lretones may be subjected, alone or in mixture with thymol, inactive piperitone, as well as inactive menthone and isomenthone of other preparation, to hydrogenation.

The solid isomenthol has not hitherto been dealt with in the literature, and therefore its behaviour has until now been entirely unknown. Its transformation into inactive menthol is of technical importance, since it enables a lay-product of low value to be converted into the valuable inactive menthol.

Example J.1OO parts ofsolid inactive isomenthol are tre "ed with hydrogen to about 180 degrees c: fade and under a pressure of about 20ull atmosph res, inpresence of parts of a nickel cat-d st, and kept in agita-' tion, until the transformation has attained a state of equilibrium, thr is to say, the maximum production of inactive menthol has l: effected, the time re mired being about 2:: hours. The inactive menthol is separated from the resulting mixture of isomers by freezing out or fractional distill; further purified by way of the esters. The isomeric mcnthols left from the'separation and purification are subjected to f rther transformation (see Example Example Q.A mixture of 50 parts of solid inactive isomenthol and 50 parts of thyinol is hydrogenated and simultaneously transformed by means of hydrogenatingcatalyst and hydrogen, as'in Example 1, and the resulting mcnthols mixture subjected to further treatment as described therein.

Example 5.;"r mixture of 33 parts of solid inactive isomenthol, 33 parts of a mixture of liquid isomeric mcnthols left over from the process described in Examgle 1, and parts of thymol or inactive menthone or inactive iscmenthone or inactive piperitone is hydrogenated with a hydrogenating catalyst and hydrogen, and simultaneously transformed, as in Example 1, the resulting menthols mixture being subjected to further treatment as described therein.

The crude inactive menthol which is obtained according to the proc lescribed may be purified by converting it into the esters or into the acid esters, for example the ester of the paranitrobenzoic acid, or the carbonate, borate, acid phthalate or succinate of the inactive menthol, crystallizing the esters and then saponifying the pure esters or acid esters, (see U. S. Patent No. 1,672,346). In this way a completely pure inactive mentholthe racemate of natural menthol--meltingpoint 3536 degrees centigrade is obtained.

l/Ve claim:

1. The process for manufacturing inactive menthol, the racemate of natural menthol, which consists in heating solid inactive isomenthol with hydrogenating catalysts and hydrogen with simultaneous agitation and separating from the reaction product inactive menthol by physical fractionation.

2. The process for manufacturing inactive -menth ol, the racemate of natural menthol,

which consists in heating solid inactive isomenthol with hydrogenating catalysts and hydrogen with simultaneous agitation and separating from the reaction product inactive menthol by fractional distillation.

3. Th process for manufacturing inactive menthol, the racemate of natural mentlml, which consists in heating solid l]1"lli'(3 isomenthol with hydrogenating cats-ysts and hydrogen with application of pressure and agitation and sepa 'ating from the rcact 111 product inactive mcnthol by physicalfra'ctionation.

l. The process for mai'nifacturing inac-' tive menthol, the racemate of natural menthol, which consists inheating solid inactive isomenthol admixed with materials dili'eri'ng from menthol only by smaller contents of hydrogen, with hydrogcnating catalysts and hydrogen with the application of pressure and agitation and separating fromthe reaction product inactive menthol by physical frac tionation.

5. The process for manu" inac tive menthol, the racemate or natura. menthol, whlch consists in heating solul mac tive isonienthol admixed with thymol, with hydrogenating catalysts and hyc "ogen with the application of pressure am. a separating from the reaction 1" tive menthol by physical fr= ctionat 6. The process for manufacturing inactive menthol, the racemate of natural menthol,

which consists in heating solid inactive isoour signatures. u

KARL SCHOLLKOPF. ARTHUR SERINI. 

